Similar to our previous TF-QKD experiment in the lab [1]}, [2]}, Alice and Bob use independent lasers: the commercial kilo-Hz continuous-wave fiber laser are locked to ultra-low-expansion (ULE) glass cavity in both sides as the light source. The linewidth of both Alice's and Bob's laser is less than 1 Hz. The central wavelength of Alice's laser is set to 1550.12460 nm. We then split the light into two, one part is to transfer quantum signal and the other one is used for wavelength locking. Instead of locking two lasers in a same room, the field experiment needs to lock the two lasers 400 km away. Alice adopts the time frequency dissemination technology [3]}, and sends the frequency locking laser through the the 84.1 dB (430 km) fiber channel with 6 EDFAs in between. The light interferes with Bob's laser at a photodiode. Based on the beating result, Bob set his laser wavelength with an acoustic-optic modulator (AOM). The relative frequency drift of the two sources is measured to be approximately 0.1 Hz/s, giving an accumulated phase difference of about \(\pi /60\)  per hour. Therefore, instead of continuing calibrating the wavelength, we only need to calibrate the wavelength difference per hour or two, which is good enough for the experiment. This can eliminate the crosstalk noise from the wavelength locking channel.
